Many common substrates in commerce, e.g., municipal sludges, which exist as finely divided particles or fibers suspended in water, need to be removed from suspension by a separation process which provides a manageable, high solids mass and a clear effluent by-product stream. Such a process results in the separation of the solids (in a high density, tractable form, ready for proper disposition) from an aqueous effluent stream which is low in solids, whereby pollution and environmental problems are abated at low cost.
Sewage sludges must be flocculated and dewatered, and the by-product stream issued into the environment, in most cases. The sludge must be dewatered sufficiently such that disposition by incineration or by land-filling is manageable and economically feasible. With regard to the latter, i.e., economic feasibility, small increases in cake solids can make a significant difference in incineration and/or disposal costs, as little as a 1% absolute increase in cake solids in large facilities handling municipal sludges where incineration is used, reducing the quantity of fuel needed for burning of the cake by several thousand gallons per day. Obviously, this is an important cost saving. In some cases, an increase in cake solids of several percent can render the mass not only manageable but high enough in solids so that it burns autogenously in incineration. Thus, with a dry enough cake the combustion process is self-supporting and the use of added fuel is significantly reduced.
In regard to landfill operations, sites are becoming overloaded, and in some localities, new sites are not available. Achieving drier cakes in the plant reduces the amount of water transported and its concomitant transportation costs and reduces the actual landfill volume requirement.
The use of mixtures of two water-soluble, cationic polymers of differing molecular weights for sludge dewatering is disclosed by Mogelnicki, et al, in U.S. Pat. No. 3,409,646 granted Jun. 1, 1966, however, the dewatering performance of such systems have not resulted in the degree of separation desired in commercial operations.
Allenson, et al, in U.S. Pat. No. 4,699,951, have also shown that clarification of wastes is accomplished by use of a mixture of two polymers, one a lower molecular weight, cationic polymer with such weights in the range of 10,000 to 1,000,000 and the other a cationic polymer with a molecular weight from 1 to 50 million. However, the use of highly branched, high molecular weight, water-soluble polymers is not disclosed by these workers. Moreover, the waste waters, which are clarified in the Allenson, et al patent, are oily wastes from refinery, petroleum and chemical processes, and the like, and are not disclosed as useful for the treatment of biological sludges.
Pech, in French Patent Application 2,598,145, describes the use of branched vinyl or acrylic addition polymers, which are made in the presence of a high activity chain transfer agent, for the flocculation of organic sludges. However, the polymers disclosed in this patent have solution viscosities of 2,200 to 3,600 mPa.s at 20% polymer concentrations, showing that they are very low in molecular weight, i.e., below one million. Such low molecular weight polymers are less effective in sludge dewatering than other systems. Also, Pech does not disclose any combined use of cationic polymers in dewatering.